For starting a steam turbine installation, the steam which is customarily generated in a waste heat steam generator is first of all not fed to the steam turbine section of a steam turbine installation, but is passed by the turbine via bypass stations and directly fed to a condenser which condenses the steam to water. The condensate is then fed again as feed water to the steam generator, or is blown out through a roof if there is no bypass station. Only when defined steam parameters in the steam lines of the water-steam cycle or in the steam lines which lead to the turbine section of the steam turbine installation, for example defined steam pressures and steam temperatures, are met, is the steam turbine brought onto line. Meeting these steam parameters is to keep possible stresses in thick-walled components at a low level and to avoid impermissible relative expansions.
If a steam turbine is stressed beyond a certain time at operating temperatures, the thick-walled components of the steam turbine, after overnight shutdowns or even after weekend shutdowns, still have high initial temperatures. Thick-walled components in this case for example are a valve housing, or a high pressure turbine section casing, or a high pressure or intermediate pressure shaft. After overnight shutdowns, which last about 8 hours, or after weekend shutdowns which last about 48 hours, the initial temperatures are typically between 300° and 500° C.
If the thick-walled components of a steam turbine installation, after a hot start or a warm start, i.e. after an overnight shutdown or a weekend shutdown, are impacted by the first available steam which the steam generator or boiler delivers, there is the risk of the thick-walled components being cooled too quickly, since as a rule the first steam has a comparatively low temperature compared with the thick-walled component.
Very large thermal stresses can result from the large temperature differences between the steam and the thick-walled components, which leads to fatigue of the material and consequently leads to a shortening of the service life.
Moreover, impermissibly high relative expansions can occur between the shaft and the casing, which can lead to a bridging of clearances.
In order to minimize the risk of excessively large temperature differences between the steam and the thick-walled components, which lead to large thermal stresses, the control valves in a steam turbine installation are currently kept closed until the steam generator or boiler delivers steam with correspondingly high temperature. These temperatures are about 50° C. above an initial temperature of individual thick-walled components. In this case, the long delay time until availability of the steam turbine installation is considered a disadvantage.